Hello Ryen.

I just encountered this bug after i got my first Great Medium Armor on the Arena Challenge for Vanguard Squads.

I first equipped it to Jessy then unequipped again to give it to Kurohime, however i got the message that i don't have enough equipment and the number went to -1

I added a screenshot of this. Don't know if this is the same with other high level equipment I don't have that many yet.

Are you using the the newest version of the game? I think you encountered an old problem, which should be fixed by now. At least I couldn't see a problem when I checked Jessy's and Kurohime's armor icons.

I used the update files for the newest version.

It seemed to me that unequipping that specific item set the counter to -1 rather than +1 but thats only speculation of course